White blood cells (WBCs, also referred to as leukocytes or leucocytes) are cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. WBCs can exist not only in the blood, but also in the lymphatic system and tissues. Some conditions can trigger a response in the immune system and cause an increase in the number of WBCs (also referred to as WBC count). Other conditions can affect the production of WBCs by the bone marrow or the survival of existing WBCs in the circulation system. Either way, these conditions can cause a change (either an increase or a decrease) of the number of circulating WBCs. Therefore, WBC counts can be a relevant physiological parameter for the diagnosis, monitoring, and treatment of various conditions including, but not limited to, bacterial and viral infections (e.g., pneumonia or meningitis), bone marrow functionality associated with chemotherapy toxicity, and haematologic proliferative processes such as leukaemia.
In current clinical practice, most of the tests to derive WBC count are performed with large-scale equipment in central clinical laboratories. Generally, these ex vivo tests are still invasive because blood samples (usually a full vial of blood is needed for each test) are collected from a patient. These blood samples are then transported, queued, and analyzed in laboratory tests, thereby taking several days to receive any results. This procedure can be burdensome for patients who need regular WBC counts or for patients with emergent conditions as well as their car. In addition, due to the ex vivo nature of conventional blood tests, there can be a certain bias of some parameters owing to the inherent differences between the measurements and the true physiological properties.